I believe the main point is that it's much, much lighter weight to run than a full Mastodon. And much simpler - most of the behavior is only a handful of lines of code in each high-level-behavior function.

That said, I'm not sure it's json-ld compatible. Seems to assume the common format, not the infinite other semantically-equivalent ways of structuring data. Though I really don't know if that's normal for mastodon-targeting stuff or if I'm missing a higher level request transformer that frames it into what they seem to assume.